The goal of these studies is to determine the basis of the extreme radioresistance of cultured TN-368 insect cells. The experimental approach is to examine several diverse aspects of the radioresponse, repair, and recovery of the TN-368 cells which all focus on their ability to survive very high doses of ionizing and non-ionizing radiation; this is to gain insight to the most critical factors responsible for this resistance. The cellular properties to be measured include colony forming ability following x-irradiation in air and nitrogen, cell cycle response, photoreactivable sector of damage, host cell and induced (Weigle-type) virus reactivation, sublethal and potentially lethal damage repair, and late recovery from heritable radiation damage. Most of the investigations will be performed by standard techniques. Late recovery will be studied by following colony forming ability, growth rate, culture density and medium alterations for the parent and cloned cell populations for several generations after x-irradiation, until normal growth conditions return. These studies will provide the direction for future research aimed at revealing the precise molecular mechanism(s) of the radioresistance of the TN-368 cells. The results of these experiments are expected to produce important information on cellular mechanisms for dealing with radiation damage which may be applied to mammalian cells. As repair mechanisms appear to play a significant role in radioresistance, these studies should be of value in better understanding mechanisms of mutagenesis and carcinogenesis.